1. Field of the Invention
This invention relates to a nitride semiconductor device, and more particularly to a nitride semiconductor device having an Si substrate as its support substrate.
2. Background Art
Switching power supply and inverter circuits are based on power semiconductor devices such as switching devices and diodes. Power semiconductor devices require high breakdown voltage and low ON resistance. However, there is a tradeoff between breakdown voltage and ON resistance, and the performance limit of the device is determined by its material characteristics. In this regard, wide bandgap semiconductors such as GaN (gallium nitride), AlGaN (aluminum gallium nitride), and other nitride semiconductors or SiC (silicon carbide) and other carbide semiconductors can be used as the material of power semiconductor devices. It is then possible to improve the tradeoff determined by the material and to realize power semiconductor devices with low ON resistance and high breakdown voltage (see, e.g., JP 2002-057158A).
On the other hand, a power semiconductor device based on such wide bandgap semiconductor material often uses an expensive SiC substrate as its support substrate, and hence tends to increase device cost. For this reason, attempts are made to use an Si (silicon) substrate, which is less expensive and having the potential to increase the wafer diameter, as a support substrate for crystal growth of nitride semiconductor layers thereon. For example, in previous studies on fabricating lateral HFET (heterostructure field-effect transistor) or other semiconductor devices, an AlN layer as a buffer layer is epitaxially grown on an Si substrate as a support substrate. A GaN layer as a channel layer is epitaxially grown on the AlN layer. An AlGaN layer as a barrier layer is formed on the GaN layer. Finally, a source electrode, a drain electrode, and a gate electrode are formed on the AlGaN layer.
In such a semiconductor device, a high voltage may be applied between the support substrate located on the backside of the device and the drain electrode located on the frontside of the device. For this reason, it is necessary to ensure a sufficient breakdown voltage in the lamination direction of the device, that is, vertical breakdown voltage. Typically, the total thickness of the buffer layer and the channel layer is increased to improve the vertical breakdown voltage.
However, in this case, the substrate and the crystal growth layer (the buffer layer and the channel layer) grown thereon are different in materials. Hence the crystal growth layer is subjected to heteroepitaxial growth. The difference of lattice constant between the substrate and the crystal growth layer limits the thickness of the crystal growth layer that can be deposited on the substrate without defects. Furthermore, even if a crystal growth layer free from defects can be formed, the difference in lattice constant and thermal expansion coefficient causes warp in the Si wafer used as the substrate and results in process-related problems. Hence it is difficult for the buffer layer and the channel layer to have a thickness large enough to ensure a sufficient vertical breakdown voltage.